The integrity of high strength bonded structures is often critical, for example in aircraft, and any failure of such a structure, like a carbon fibre wing, for example, could be critical to the safety of the aircraft. The problem of assessing bond integrity may be equally important for structures which are not necessarily safety critical but which are either extremely expensive, such as large wind turbines, or which are expected to perform for long periods reliably and which may be difficult to access, such as building structures.
Such bonded structures may be of fibre-reinforced composite material or may be metallic structures, such as bonded aluminium vehicle bodies.
It is known to place fibres within fibre-reinforced composite components whose failure, due to impact damage for example, may send an electrical signal to a processor to alert an operator. To date, however, no such solution is known for assessing the integrity of bonds between parts of a structure. It is envisaged that a particular difficulty with such a system would be getting damage information from the bond as the bond is, by definition, located between parts of a unified structure. In addition, any edges of such a bonded joint will often be inaccessible owing to the way in which the component is structured. In assemblies containing bonded joints, those joints may butt up against one another making the edge of the joint inaccessible. Where the bond edge is accessible, an optical fibre cannot emerge through it without protection as it will be liable to break. Protection schemes, such as tubing, are likely to adversely affect the strength of the bond, making such an approach inappropriate.
Bonded structures likely to cause particular difficulty with both inspection and access, for optical fibre damage detection, are airframe structures, in particular wings and fuselages, where a clean aerodynamic surface is required on one side of the structure and where they may be no accessible edge to the structure from which to access the ends of optical fibres embedded in the structure.